Why can't everyone just have their beliefs?
Questions like this are usually presented as appeals to emotion, and are some of the hardest questions presented to answer well.
One reason to be questioning a belief system would be that they may not be thinking rationally or reasonably, and therefore critical thinking should be aplied.
Another answer for this question may be what religion has caused and acheived throughout history, not only has theism spread some of the worst morals and caused many griefs throughout history, it slows the advance of science. Science, as many people will know, provides all of the worlds technology. It advances medicine which saves millions on a yearly basis, and has made almost everything in the common household. by slowing the scientific method, religion is slowing the process of helping mankind better itself. As well as theists in general, there are evangelicals such as Kent Hovind who actually try to stop and destroy science.